The present invention relates to a corona charging device for depositing charge on an adjacent surface. More particularly, it is directed to a corona charging arrangement usable in a xerographic reproduction system for generating a flow of ions onto an adjacent imaging surface for altering or changing the electrostatic charge thereon.
In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to precharging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, the conditioning of the imaging surface prior to, during, and after the deposition of toner thereon to improve the quality of the xerographic copy produced thereby.
Both d.c. and a.c. type corona devices are used to perform many of the above functions.
The conventional form of corona discharge device for use in reproduction systems of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating d.c. voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an a.c. corona generating potential is applied to the conductive wire electrode and a d.c. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
Several problems have been historically associated with such corona devices. A first problem has been inability of such devices to deposit relatively uniform negative charge on an imaging surface.
More specifically, when a corona electrode in a device of the above type is biased with a negative corona generating potential, the charge density varies greatly along the length of the wire resulting in a corresponding variation in the magnitude of charge deposited on associated portions of an adjacent surface to be charged. This problem is visually verified as glow spots along the length of the corona wire when negative corona potentials are applied as contrasted to the more uniform corona glow when positive potentials are applied. More basically, the nonuniformity is believed to result from the fact that negative corona is initiated by high field stripping of electrons from the surface of the wire and sustained in large measure by secondary emission processes at the surface. This secondary emission process is easily affected by surface contamination which typically occurs from chemical growths on these surfaces. Positive ion bombardment also is believed to contribute to the nonuniformity problem by partially cleaning portions of the wire, which cleaned portions become emitters or relatively high current with respect to the remainder of the wire.
The above problem of nonuniform negative charging is addressed in U.S. Pat. Nos. 3,813,549 and 3,789,278 and it is suggested therein that various thin dielectric coatings be applied to the metallic wire electrode of the conventional corona charging arrangements while applying a negative d.c. potential to the corona electrode in order to lessen such nonuniformity. Improved uniformity, it is suggested therein, may result from the localized current limiting provided by the thin dielectric.
More specifically, U.S. Pat. No. 3,789,278 suggests the use of a thin high resistivity coating spread uniformly over the surface of a valve metal wire electrode. U.S. Pat. No. 3,813,549 suggests the use of a thin dielectric coating over the surface of a metallic wire electrode. In both of the above arrangements, a d.c. potential is used to energize the wire and a d.c. current through the coatings is used to deposit charge on an adjacent surface. The coatings suggested for use in these patents, while being made of dielectric materials, must of necessity be sufficiently thin to permit the passage of d.c. charging current therethrough.
A further problem associated with conventional corona discharge devices employing a conductive wire is a result of the fact that corona glow is associated with a region of high chemical reactivity where chemical compounds are synthesized from machine air, which results in chemical growths being built up on the surface of the wire. These chemical growths, after a prolonged period of operation, degrade the performance of the corona device. Since free oxygen and ozone are produced in the corona region the corona electrode must of necessity be highly oxidation resistant. The above problem of chemical growth build-up on the wire has been addressed by the provision of wire materials which are less subject to chemical attack. While this has reduced the problem, such materials have substantially increased the cost of corona devices.
A still further problem associated with corona discharge devices operating in a xerographic environment results from toner accumulations on the surface of the corona electrode. The spots of accumulated toner, being dielectric in nature, tend to cause localized charge build up on the interior surfaces of the shield which produces current nonuniformity and reduction in corona current. Localized toner accumulations on the insulating end blocks which support the wire electrode also cause sparking.
A still further disadvantage of prior art corona discharge devices is the fact that d.c. charging current is drawn through the wire and passes therefrom along either of two parallel paths. The first path includes the air space between the corona electrode and the surrounding conductive shield, and the shield itself, which is usually grounded. The second path includes the air space between the corona electrode and the surface to be charged, the surface itself and the grounded substrate on which the surface is carried. Since the surface to be charged rests directly on a grounded substrate, and since this arrangement has the obvious advantage of not having to electrically isolate the photoconductive drum above ground, it is not possible to measure directly the charging current flowing to the surface to be charged. The charging current to the surface can be determined only when both the total current to the wire and the current drawn by the shield are known (assuming a directly grounded photoconductor support). The problem is compounded when several corona generators are operated from a common supply. Such an electrical arrangement has conventionally required either a complex electrical arrangement or a less direct method (electrometer) to sense and control currents accurately. An improved system which operates to more easily compute corona charging current in the above noted environment is disclosed in copending patent application, Ser. No. 572,683, filed Apr. 28, 1975, and commonly assigned. The arrangement disclosed in the above application is necessitated by prior art corona charging arrangements wherein d.c. corona current drawn by the corona electrode is delivered to both the shield and the surface to be charged.
Yet another problem has been associated with the use of corona generators energized by an a.c. source for the purpose of reducing to zero or neutralizing the charge on a surface. This is a well known process and relies on the voltage sensitivity characteristics of corona generators. More specifically, the amount and polarity of charge delivered to a charged surface is a function of the polarity and magnitude of the charge on that surface. Thus, if the surface having a net positive charge on it is exposed to an a.c. corona generator, the negative current pulses delivered to the surface will be slightly greater than the positive pulses. After a number of cycles this action tends to reduce the positive charge on the surface. In order to completely neutralize the charge, i.e., to reduce the charge to zero, the corona generator must have the characteristics of delivering a zero d.c. current when exposed to surface with no net charge thereon. This latter characteristic is not generally an inherent property of conventional corona generators used in xerographic machines. One prior art solution to this problem is to place a d.c. bias on the corona electrode about which the a.c. corona generating voltage varies. Another proposed solution, suggested by U.S. Pat. No. 3,714,531, is to selectively place different resistances in series with the corona electrode during alternate half cycles, thus equalizing charge generation. Both solutions have the disadvantage of requiring additional external biasing components. In addition, the charge output of such arrangements tends to change significantly with temperature and humidity.
A further disadvantage of prior art corona devices is the inflexible nature of their characteristic output. As is well known in the art, corona devices of the type disclosed herein produce charging current (Ip) of a magnitude which is a function of the potential on the charge accepting surface (Vp).
A curve relating charging current to the potential of an adjacent charge accepting surface at a given corona producing voltage will be referred to hereinafter as the I-V curve and is important in determining the effect of a corona device on the surface to be charged. For many applications, it is desirable to adjust the slope of the I.sub.p -V.sub.s curve and the location of the Ip=0 intercept on this curve. In prior art devices adjustment of the slope of the I-V curve was not possible without a substantial change in the Ip=0 intercept.
It would, therefore, be desirable to provide a corona device having a I-V curve which was easily adjusted in slope.
Yet another problem of lesser consequence than those previously alluded to is the vibration associated with the suspension of a relatively thin metal filiment in a high electric field employed in corona discharge devices.